1. Field of the Invention
This invention relates to polyurethane resins obtained from aromatic diisocyanates, which are soluble in apolar or slightly polar solvents, and to a process for their preparation from relatively high molecular weight dihydroxyl compounds, diisocyanates, diol chain lengthening agents and optionally monofunctional chain breaking agents wherein 2,2,4-trimethyl-pentanediol(1,3) is used as all or part of the chain-lengthening agent. This invention is also directed to the use of these polyurethanes for the production of films or coatings.
2. Description of the Prior Art
Linear polyurethanes obtained from dihydroxyl compounds and diisocyanates have long been known and numerously described in the literature. They generally consist of segmented high polymers in the form of chains built up of macrodiols, diisocyanates and low molecular weight chain lengthening agents. One class of these polymers includes the polyurethanes prepared from aromatic diisocyanates, in which the chain lengthening agents used consist almost exclusively of diols. Such polyurethanes are generally only soluble in solvents containing at least a proportion of highly polar substances, such as dimethylformamide, dimethylsulphoxide or N-methylpyrrolidone. Such solvents are not toxicologically and ecologically harmless, particularly when handled inexpertly, and require the use of costly installations for their recovery. Furthermore, they are unsuitable for use in the increasingly important field of lacquering synthetic resins since they unduly attack the surfaces of synthetic resin articles and to some extent even dissolve them. This results not only in changes in the nature of the surface (smoothness and gloss), but also in loss of dimensional stability or of certain structures (e.g. in the case of foams having an integrated non-cellular skin). For many fields of application, there is therefore presently a demand for the use of solvents which are non-corrosive and harmless in other respects, such as aromatic hydrocarbons, ketones and esters. Unfortunately, most of the "aromatic" polyurethanes are insoluble or not completely soluble in such solvents. There has therefore been no lack of attempts to modify these polyurethanes in some suitable manner to enable them to be completely dissolved in less polar solvents.
Thus, the aromatic polyurethanes disclosed in DAS No. 1,106,958 are soluble in methyl ethyl ketone or ethyl acetate if diols corresponding to the following general formula ##STR1## (A=alkylene, B=H or C.sub.1 -C.sub.5 group) have been used for their synthesis. The resulting polyurethanes, however, only contain a limited proportion of diols, based on their macrodiol content (0.1 to 3 mol to every 1 mol) and the products obtained have insufficient hardness for many purposes of application. If the proportion of chain lengthening agent is increased, the polyurethanes obtained have satisfactory properties but their solubility in the above-mentioned solvents is insufficient.
DAS No. 1,106,959 discloses polyurethanes obtained from macrodiols and aromatic polyisocyanates which are chain lengthened with straight chained diols having 2 to 10 carbon atoms and are soluble in DMF, cyclohexanone, tetrahydrofuran or dioxane. In this case, only 2 mol of chain lengthening agent may be used to 1 mol of macrodiol if clear solutions are still to be obtained.
According to DAS No. 1,301,124, similar polyurethanes are prepared in boiling chlorinated aromatic compounds such as mono- or dichlorobenzene and then obtained as solid product by precipitation from the liquid reaction mixtures.
Lastly, in DAS No. 2,442,763 there are disclosed linear polyurethanes obtained from macrodiols, 4,4-diisocyanato-diphenylmethane and its derivatives and low molecular weight, branch chained diols, in which the last component is preferably 2,2-dimethylpropanediol(1,3) ("neopentyl glycol") and which are distinguished by the fact that in spite of their high content in chain lengthening agent (3.5 to 15 mol of neopentyl glycol to 1 mol of macrodiol), they are soluble in cyclic ethers such as tetrahydrofuran or dioxane and cyclic ketones such as cyclohexanone. Very hard coatings can be produced with these polyurethanes.
Solvents such as cyclic ethers or ketones offer some advantages over highly polar solvents such as dimethyl formamide, dimethyl sulphoxide or N-methylpyrrolidone. They are less corrosive to synthetic resins and physiologically less harmful. On the other hand, cyclic ethers have a tendency to form dangerous peroxides, and polyurethanes obtained from these ethers and cyclic ketones are not universally suitable for use on synthetic resins. Although such solutions may be blended with solvents such as non-cyclic ketones, esters or aromatic compounds, this does not eliminate their corrosive action on synthetic resins.
Only polyurethanes with a low chain lengthening agent content have therefore hitherto been soluble in these very useful aromatic hydrocarbons as indicated above. For many fields of application, these polyurethanes are unusable because they have insufficient hardness. It is precisely for particularly delicate substrates such as carrier tapes for magnetic recording materials that it would be particularly desirable to be able to apply hard, resistant polyurethanes from a solvent or solvent mixture containing aromatic hydrocarbons.
It was therefore an object of the present invention to provide polyurethanes with great hardness and mechanical wear resistance which could be applied from a solvent or mixture of solvents containing an aromatic hydrocarbon and would be suitable for use on substrates which are sensitive to solvents.
It has now surprisingly been found that such polyurethanes may be obtained by using 2,2,4-trimethylpentanediol-(1,3) ##STR2## as chain lengthening agent in a polyurethane based on macrodiols, aromatic diisocyanates and chain lengthening agents.
The polyurethanes thus obtained may be dissolved in purely aromatic hydrocarbons, in ketones and optionally esters or in any mixtures of these solvents.
Furthermore, these polyurethanes may be prepared from the start in these solvents or mixtures thereof. Clear, viscous solutions which are free from gelled particles and have normal flow properties are obtained in both cases. It is surprising to find that in spite of the excellent solubility of these polyurethanes, especially in aromatic hydrocarbons, the films obtained from them do not differ markedly in their surface hardness from films of analogous polyurethanes prepared according to the known art from 2,2-disubstituted 1,3-propanediols. As already mentioned above, it is precisely these properties of surface hardness which in the past had to be forfeited if improved solubility was to be obtained.